Add const to RenderProcessHostImpl::storage_partition_impl_.

It never changes.

Change-Id: I04f9f7d6d639fc4d67108b1f4741f8a334a1be56
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1837562Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702673}

content/browser/renderer_host/render_process_host_impl.h

@@ -871,7 +871,7 @@ class CONTENT_EXPORT RenderProcessHostImpl
   BrowserContext* const browser_context_;
 
   // Owned by |browser_context_|.
-  StoragePartitionImpl* storage_partition_impl_;
+  StoragePartitionImpl* const storage_partition_impl_;
 
   // Keeps track of the BindingIds  returned by storage_partition_impl_->Bind()
   // calls so we can Unbind() them on cleanup.